Electronic control of machines frequently requires information about the machine operation to be furnished to the control circuit. The absolute angular position of a rotating shaft and the shaft speed are data which are used in many control methods. In particular, ignition or fuel injection control of internal combustion engines makes use of information about the engine speed, even on an instantaneous basis, and the cylinder position. Then it is possible for the fuel or ignition circuit to apply fuel or ignition voltage to the proper cylinder at the precise time for optimum performance.
It is known to use magnetic pickups with a gear or other toothed wheel to sense the rotation of an engine shaft such as a cam shaft or crank shaft, and to use a special index tooth or a separate pickup to establish the shaft position once each revolution. Similar arrangements using optical pickups with optical encoder disks are also known. Such disks have tracks of black and white patterns or transparent and opaque patterns to furnish binary data. Optical encoder disks with many tracks furnish several bits of digital information which makes accurate position information possible. This requires many optical detectors, as well, thereby making such a system expensive. Such a device is shown in the U.S. Pat. No. 3,757,755 to Carner which is applied to engine control. A simpler apparatus is shown in U.S. Pat. No. 4,604,725 to Davies et al; a code track and a timing track are used along with two, or preferably four, fiber optic pickups to obtain position information only. Both of these patent disclosures specify binary codes on the encoder disk. This is the usual case since binary codes are easy to read and traditionally are thought to be readily interfaced with digital control circuitry. On the other hand binary codes using "1" and "0" as information bits are limited in the amount of information content.
The need for sufficient information for accurate control purposes has led to complex designs as discussed above, yet it is generally acknowledged that simple designs are superior, if they accomplish the required function, because they are generally less expensive, more trouble free, and easier to maintain. In this invention it is recognized that by departing from binary codes, information may be compacted so that a single encoded track carries both speed and angular position information, and yet traditional binary digital circuitry can decode the information quickly and efficiently. Decoding the information will require processing, usually by a microprocessor shared by other functions such as engine spark and fuel control, and thus it is highly desirable that the processor burden be minimized.